The present invention relates to the art of image calibration. It finds particular application in conjunction with the calibration of video to photographic film cameras, video monitors, and the like. However, it is to be appreciated that the invention has broader applications in the calibration of electronic imaging, video electronics, diagnostic photographic imaging equipment, and the image representations thereof.
Techniques for generating electronic images from computer memory on a video display device are well understood in the art. See The McGraw-Hill Computer Handbook, Section 20 (1983).
Heretofore, photographic cameras and video monitors have commonly been interconnected with medical diagnostic imaging equipment. The cameras included an internal video display for converting electronic image signals into a suitable display for exposing photographic film. An appropriate lens, shutter, aperture, and the like were disposed between the internal video display on a film transport or carriage. Upon initial installation, the brightness and contrast of the internal display, the shutter speed and the aperture were set or adjusted by trial and error. Typically, the density range of photographic film extended from 0.16 to 2.80. However, the extremes of this range had a non-linear response. To avoid the non-linear region, the internal video monitor and the camera exposure settings were commonly adjusted until white regions had an optical density of about 0.24 or greater and the black regions had an optical density of 2.26 or less. The gray scales were then determined by the optical densities at equal intervals therebetween. This calibration required trial and error and the use of a densitometer to determine that the photographic images, in fact, had the optical densities required by the calibration standards.
After initial installation, one could check the calibration of the camera by taking one or more additional photographs and examining the optical density of the black, white, and gray scale regions to determine if the optical density still met the calibration standards. However, optical densitometers were relatively expensive and complex to operate, rendering the procedure for checking the calibration time consuming and expensive.
Once the camera was calibrated, an analogous calibration procedure was performed on the video monitor. Using appropriate optical intensity measuring equipment, the brightness and contrast of the video monitor were adjusted until calibration specifications are met. This adjustment was complicated by the differences in brightness levels in the video and photographic images. When the photographic image was viewed on a conventional light box, the white areas had a light intensity of about 600-800 foot lamberts. Whereas, the same white areas on a video monitor had a light intensity of about 10-20 foot lamberts.
Again, the optical intensity measuring equipment was relatively expensive and cumbersome to use. Such video monitor calibrations were further expensive in that expensive medical diagnostic equipment and optical intensity equipment were tied up for extended durations.
Due to the complexity of adjusting the video monitors and because the brightness and contrast controls were commonly accessible to the user, there was a tendency for the radiologist or other user to adjust the brightness and contrast until a pleasing image was attained. This manual adjustment of the video monitor commonly resulted in gray scale and other image differences between the image viewed on the video monitor and the permanent record made on photographic film. These differences between the on-line diagnostic image and the permanent photographic diagnostic images could lead to errors in the medical diagnosis.
The present invention contemplates a new and improved calibration procedure and calibration pattern which eliminates the above referenced problems and others.